Cheap readers will only read 'some' codes, and the dash test doesn't always help either. My reader was £80 and that doesn't pick up all codes. They will pick up basic stuff and give a vague description of the area the problem could be in. But even then you could be chasing your tail. Best to get it to a garage who are more likely to be able to pinpoint the problem.

A decent garage with testing equipment would be cheaper than ford, BUT, ford is for ford, and sometimes only this will do, so its down to personal choice really, take your chance at a local garage, or go to ford and pay that bit extra.